Why there is no light coming inside from the window
Besides the glass issue pointed out by @Timaroberts your model has lots of problems The most important one is that Materials created in blender internal will not work on cycles, you need to create new materials using cycles shaders.

Then: Unconnected nodes have no effect.



Never connect nodes on a redundant loop (see the red line?)



Then you have a big box with a solidify modifier surrounding all of the "room", so that even if your materials were correctly set, that box would not let light through the glass anyway...



Fix all of that before you can have volumetrics... to set it up read this post:

Atmospheric lighting in cycles